Method and apparatus for preparing natural gas for liquefaction



June 21, 1960 w. L. MORRISON 2,941,374

METHOD AND APPARATUS FOR PREPARING NATURAL GAS FOR LIQUEFACTION Filed Aug. 16, 1954 INVENTOR, WILLARD L. MORRISON y Parker a Carter ATTORNEYS United States Patent V METHOD AND APPARATUS FOR PREPARING NATURAL GAS FOR LIQUEFACTION Willard L. Morrison, Lake Forest, 111., assignor, by mesne I assignments, to Constork Liquid Methane Corporation, a corporation of Delaware Filed Aug. 16, 1954, Ser. No. 450,131

7 Claims. (CI. 62-23) 428,526, filed May 10, 1954, wherein the gas is cooled,

expands and does work in an expansion turbine, is exhausted from the turbine at substantially atmospheric pressure and at such a low temperature that part of the gas is liquefied. The liquefied gas is separated from the cold unliquefied gas and discharged from the system.

One object of my invention is to provide a new and improved method and apparatus for using the pressure of gas from a gas well to cool the gas prior to cleaning it, before liquefaction, to remove from the gas those components which might interfere with liquefaction or might otherwise be undesirable in a liquefied. product.

Another object, is to more effectively use the reduction in temperature resulting from the reduction of pressure of the gas, to facilitate the removal from the gas of those impurities which might interfere with the liquefaction.

Another object, is to provide an efiective method and apparatus for the recovery of those condensates condensed as a result of the above mentioned temperature reduction.

Another object of the invention, is to use the gas, entering the system at high pressure, for a motive fluid, which will expand and do external work to lower the temperature of the gas so that it may be used as a refrigerant to cool the gas after clean up and before the main liquefaction expansion.

Another object is to cause the gas entering the system to expand and do external work with resultant temperature of the gas lower than would be obtained by expansion alone. The power generated as the result of such work, being used to raise the pressure of the gas back up to the desired pressure for expansion and resultant liquefaction of the gas in the main expansion turbine.

A still further object of this invention is to make use of natural gas at casing head pressure in an expansion system for liquefaction wherein the temperature of the gas at the pressure for expansion is below that resulting from the pressure drop between the casing head pressure and the pressure employed for expansion. It is a related object to make use of the forces available during processing in the manner described for most efficient removal of undesirable ingredients from the gas prior to such expan- SlOIl.

The gas from the well or other source of supply enters the system at high pressure, that, perhaps 2000 or 2500 p.s.i. drives an expansion engine, preferably in the form of a free running turbine and expands through it into a separation vessel or chamber on its way to the main expansion turbine. In the event that the drop in temperature .upon expansionis sufiicient to cause solidification, such as freezing of any of the ingredients in the gas, it is desirable tointroduce a liquid into the gas,'which will "2,911,374 Patented June 21, 1960 ice 2 tion of glycol or other polyhydric alcohol prior to the expansion engine and prevent icefonnation.

Utilization is made of the cold in the gas from the first expander to reduce the temperature of the gas advanced to the main expander by passing the cold gas in heat exchange relation therewith. At the same time the cold gas exhausted from the first expander is increased to a temperature made suitable for eflicient operation of clean up equipment for removal of such undesirable ingredients as carbon dioxide, hydrogen sulfide, and other acid gases and moisture. From the heat exchanger, the exhausted gas is cycled through a blower which utilizes the energy made available by the gas during expansion through the first expansion engine to increase the pressure of the gas to a more desirable level for final expansion, but to a pressure below its initial pressure.

Usually the heat of compression will raise the temperature of the gas to a level above that for most eflicient operation of the cleanup equipment. In that event, a distinct improvement is achieved by the use of a water cooler or the like for extraction of heat from the gas prior to processing through an amine contactor and dehydrator.

The gas from the cleanup cycle returns through the gas heat exchanger, being cooled by the cold gas from the separation vessel and enters the main turbo expander where it is liquefied.

The use of the free running turbo-expander-fan, where the gas does work in and expands through theturbine with reduction in temperature and pressure and generates power to operate the fan makes it possible to get a substantially greater temperature drop and more eflfective cooling than can be obtained by the use of a simple expansion valve expanding into a separation vessel. The fan or blower raises the pressure and temperature of the gas on its way to the cleanup stage but, of course, not up to the pressure at which the gas entered the free running turbine.

Other objects of my invention will appear from time to time throughout the specification and claims.

My invention is illustrated more or less diagrammatically in the accompanying drawing wherein like parts are indicated by like characters throughout.

1 is a duct for gas which may Well come from the well head and have the pressure resultant therefrom though the gas might be supplied from-other suitable high pressure source. 2 is an injection duct which feeds a suitable liquid such as glycol in any suitable manner into the gas stream to prevent freezing or solidification of ingredients in the stream of gas being discharged into the separation vessel 4.

The glycol and other condensibles will condense out in the vessel 4 and may be discharged through the duct 5. The disposition of the glycol and other condensates inthe tank form no part of the present invention and are not further illustrated.

6 is a duct leading from the vessel 4 to the heat exchanger 8. The gas passes through the heat exchanger 8, the pipe 9, and is drawn through and compressed by the fan 10, passes through the duct 11, and heat exchanger or after cooler 12, to lower, if necessary, the temperature of the gas, having been warmed by passage through the heat exchanger 8 and the fan 10, to a point at which it can suitably enter the amine contactor. The pipes 13, 14 illustrate the water supply to the after cooler 12. Further details being conventional are not illustrated.

15 is a duct leading from the heat exchanger 12 to the clean up equipment 17, '17a. The details of the clean well, an amine contactor, a dehydrator and other'clearr up apparatus; which remove acid gases and other impurities, are not further illustrated. Suffice it to say that the gas from the clean up cycle passes through the duc 8, o e. ga he t e c ange h ou h co l, 1. he e. to. the du t. :9 h ma n p nd r. u fa ion.

Assuming for example that the gas, perhaps from the I well or from other suitable source is at 2000 p.s.i.a. and 7Q F. as it enters the primaryexpansion stage, the free running turbine 3. The exhaust from the turbine will'enter the separation vessel 4 at 750- p.s.i.a. and minus 30 F. What happens to the condensates, if any, is no part of the present invention and so far as the present invention is concerned the main purpose of the vessel 4 is to provide a closed space into which the cold gas may expand after exhaust from the turbine.

The cooled gas passed in heat exchange relation with the cleaned gas on its way to the main expansion liquefaction turbine 20 will issue from the heat exchanger at about'740 p.s.i.a. and 70 F. for admission to the turbine driven free running compressor fan 10. The fan raises the gas pressure to about 1050 p.s,i.a. and the heat of compression will raise the temperature of the gas to about 130 F. After passing through the intercooler 12 the gas may be at 1040 p.s.i.a. and 100 F. as. it enters the-clean up Zone 17 and 17a.

After passing through the various stages, whatever they may be, of the clean up cycle or zone, the gas will approximate 101 p.s.i.a. and 81 F. as it enters the gas heat exchanger 8 where it will be cooled by the gas expended in the preliminary expansion stage 3, to F.--a suitable pressure and temperature for the final expansion liquefaction stage in the expander turbine 29.

The result of the above sequence is that the entry pressure of the gas is effective to provide a refrigerating effect greater than that resulting from a simple expansion, without external work, of the gas. This provides a lower temperature to promote a more eificient condensation and separation of higher hydro carbons if that is desired, in the separation vessel. The return, after use of the gas as a refrigerant, increases pressure and temperature of the gas toa point suitable for passage through the clean up cycle where acid gases and other undesired elements can be removed from the gas before liquefaction.

Thus, after the gas has again been cooled, it will be at the desired pressure and temperature for liquefaction.

While the subject matter of this invention is especially adapted to the liquefaction of natural gas at Well pressure, largely containing methane, it will be obvious that -the method herein disclosed may be applicable to the liquefaction of other gases and from other sources and at difierent pressures, and in such case the disclosure herein may be taken as illustrative of possible modifications.

While it is believed to be inventive to make use of a sequence of operations employing separate expansions while doing work in the described expansion turbines, use may be made of means other than the final expander turbine 20, for reducing the cold, clean gas or uncleaned, in whole or part, to a liquefied state, such as by a cascade system, expansion valve employing the Joule-Thompson principle or the like.

" I claim:

1. The method of liquefying natural gas which consists in causing the gas at high pressure to expand and do external work with resultant temperature drop in. a primary expansion; turbine, withdrawing condensibles in liquidform-from the, cold gas warming the cold gas by heat exchange, using the power developed by the expansion of the gas to raise the pressure and temperature of the gas above the pressure and temperature of the exhaust from the expansion turbine, cooling the gas after its pressure has been raised, cleaning the gas by removing from it such elements as are not desirable with the liquefied gas, especially the acid gases, cooling the cleaned gas by heat exchange with the cold gas after the primary expansion. stage and then. expanding the cooled cleaned gas and causingit to do external work in a secondary expansion stagewith resultantliquefaction off-part of the gas. t

2. In the method for the liquefaction of natural gas composed chiefly. of: meth'aneandfl containing minor proportions of higher boiling hydrocarbons and gaseous impurities, the step of expanding the gas in a first expansion engine from casing head pressure to lower pressurewith accompanied reduction in the temperature of the gas, removing condensed components from the gas exhausted from the first expansion engine, heating the ex-. haust to a higher temperature for passing through cleanup equipment for removal of undesirable ingredients in the gas from the well, removing the gaseous impurities from the gas, and then passing the clean gas in heat exchange relation with the cold exhaust from the first expansion engine to reduce the temperature of the clean gas while heating up the cold gas from the first expansion engine and then expanding thecold clear gas through a second expansion turbine. i

3. Inthe method for liquefying of natural gas composed chiefly of methane and containing minor proportions of higher boiling hydrocarbons and gaseous impurities, the" step of expanding the gas while doing work in a first expansion engine from casing head pressure to a lower pressure accompanied with a reduction in the temperature ofthe gas, removing condensed components from the gas exhausted from the first expansion engine, compressing the exhausted gas to a higher pressure by the power generated by' the gas in the first expansion engine, removing gaseous impurities from the recompressed gas, utilizing the cold in the gas from the first expansion to reduce the temperature of the recompressed gas, then expanding the cooled, recompressed and cleaned gas while doing work through a second expansion engine after being cooled by the exhaust from the first expansion engine to liquefy the gas.

4'. In combination, a source of combustible gas under pressure, a free running turbo-expander adapted to generate power as the gas expands 't-herethrough, a vessel adapted to receive the resultant cold gas, a gas heat exchanger through which the cold gas passes and wherein it is warmed, a compressor driven by the turbo-expander,

means for conducting the warmed gas to the compressor,

wherein the gas is raised in pressure and temperature, an

after cooler whereinthe gas is again cooled, gas clean up means, through which the cold gas passes, means for passing the cleaned gas through the gas heat exchanger where it is cooled by the gas from the vessel, a main liquefaction expander turbine and means for conducting the gas from the gas heat exchanger thereto for expansion and liquefaction- 5; The method of liquefying natural gas which consists in causing the gas at high pressure to expand to an intermediate pressure and do external work with resultant temperature drop in a primary expansion stage, withdrawing condensibles from the cold gas from the primary expansion stage, utilizing the power released from the primary expansion stage to raise the pressure of the gas above the measure of the exhaust from the primary expansion stage after the condensibles have been re.- moved, extracting heat from the gas after its pressure has been raised, and causing the recompressed and cooled gas to expand further to a pressure still lower than the pressure ofthe exhaust from the primary expansion stage doing work in a subsequent expansion stage with resultant lowering of the temperature of the gas for liquefaction of a portion thereof.

6. The method of liquefying natural gas composed chiefly of methane and containing minor proportions of higher boiling hydrocarbons and acidic gaseous impurities which consists in causing the gas at high pressure to expand to intermediate pressure and do external work with resultant temperature drop in a primary expansion stage, withdrawing condensibles of higher hydrocarbons from the cold gas from the primary expansion stage, cleaning the gas by removing from it such elements as are not desirable with the liquefied gas, especially the acid gases, causing the cleaned gas to expand further and do external work in a subsequent expansion stage with resultant liquefaction of a part of the methane of the natural gas, and utilizing the work released from the primary expansions for recompression of the gas exhausted from the primary expansion stage.

7. The method of liquefying natural gas which consists in causing the gas at high pressure to expand to an intermediate pressure and do external work with resultant temperature drop in a primary expansion stage,

withdrawing condensibles from the cold gas, utilizing the power released from the primary expansion stage to raise the pressure of the gas from the primary expansion stage after the condensibles have been removed, passing the recompressed gas in heat exchange relation with the cold gas from the primary expansion stage to cool the recompressed gas, and causing the cooled gas to expand further to a pressure below the pressure of the gas exhausted from the primary expansion stage while doing external work in a subsequent expansion stage with resultant cooling for liquefaction of a portion of the natural gas and utilizing the work released from the subsequent expansion stage for recompression of the gas.

References Cited in the file of this patent UNITED STATES PATENTS 1,696,558 Van Nuys Dec. 25, 1928 2,030,509 Fr'ankl Feb. 11, 1936 2,617,276 Gard et a1. Nov. 11, 1952 2,679,145 Hagen May 25, 1954 

